Conventionally, there have been known several methods for producing a fatty acid salt through reaction of a fatty acid with a metal oxide or a metal hydroxide; e.g., (1) the wet direct method, (2) the double decomposition method, and (3) the dry direct method (disclosed, for example, in “Properties and Applications of Metallic Soap,” edited by Tokiyuki YOSHIDA et al., published by Saiwai Shobo (1988)).
Among these methods, (1) the wet direct method employs neutralization reaction in which a fatty acid in liquid form is added dropwise, with stirring, to a dispersion of a metal hydroxide micropowder in water, to thereby obtain a fatty acid salt. In (2) the double decomposition method, a fatty acid is added dropwise, with stirring, to an aqueous dispersion or aqueous solution of a hydroxide of an alkali metal such as sodium or potassium, to thereby synthesize alkali soap, and an aqueous solution of a metal chloride or sulfate is added dropwise to an aqueous solution of the alkali soap so as to react the alkali soap with the metal, to thereby obtain a fatty acid salt. When method (2) is employed, the overall reaction can be completed highly stoichiometrically. Therefore, method (2) has been a mainstream method among conventional fatty acid salt synthesis methods.
In (3) the dry direct method, a liquid fatty acid is sufficiently mixed with micropowder of a metal oxide or a metal hydroxide through high-speed agitation or a similar technique, and a predetermined amount of water is added to the resultant mixture, thereby inducing simultaneous reaction. As compared with the above methods (1) and (2), method (3) is advantageous in that no filtration step is required and that a drying step requiring a large amount of heat can be simplified.